Transmission lines are commonly employed to communicate signals between various portions of an electronic system. For example, coaxial transmission lines, waveguides, and even parallel arrangements of metallic conductors are typically employed as transmission lines in such systems. Increasingly, fiber-optic transmission lines are used instead of conventional metallic transmission lines to communicate signals in electronic systems due to the generally higher noise immunity and lower signal attenuation properties obtainable in such lines. Additionally, fiber-optic transmission lines are generally thinner and lighter than metallic conductors of comparable capacity.
In all systems employing transmission lines, difficulties may arise due to degradation of the line resulting from physical damage, aging, poorly matched and/or damaged connectors, or for other reasons. In practice, difficulties with transmission lines are frequently difficult to detect and diagnose, particularly in electronic systems where only a single terminal end of the transmission line is accessible. Although a number of different methods are available to detect and diagnose transmission line difficulties, one useful and commonly employed method is time domain reflectometry.
FIG. 1 is a partial elevational and schematic view of a known apparatus 10 that is operable to perform time domain reflectometry on a fiber-optic transmission line assembly 12. The assembly 12 generally comprises an interconnecting communications element that is configured to communicate electromagnetic signals between various electronic devices (not shown in FIG. 1). The assembly 12 includes a plurality of generally abutting fiber-optic segments 16 that are operatively coupled by one or more connectors 18 positioned at selected locations along a length of the assembly 12. In order to achieve favorable coupling efficiency, the connectors 18 are generally configured to approximately conform to the optical characteristics of the segments 16. Consequently, the connectors 18 typically closely approximate the optical impedance present in the segments 16. Due to defects, damage, or even misalignment of the connectors 18, undesired impedance “bumps” may be present in the fiber-optic transmission line assembly 12 that adversely affect the transmission of optical signals along the assembly 12. The fiber-optic transmission line assembly 12 may further include one or more defects 20 located at various positions along the assembly 12, such as cracks, defective splices, or other similar discontinuities, which may further degrade the optical performance of the assembly 12.
Still referring to FIG. 1, the apparatus 10 will now be described with reference in particular to the detection of optical defects or discontinuities in the assembly 12. The apparatus 10 includes an optical signal source 22 that emits one or more relatively short-duration pulses of optical energy 24 towards a partial mirror 26 that permits at least a portion of the optical energy 24 to be transmitted into a terminal end 14 of the assembly 12, which propagates along the length of the assembly 12. When the optical energy 24 encounters a defect or discontinuity in the assembly 12, reflected energy 28 generally proportional to the magnitude of the impedance mismatch presented by the discontinuity propagates backwardly towards the terminal end 14 of the assembly 12. The reflected energy 28 then emerges from the terminal end 14 and is substantially reflected by the partial mirror 26 and into an optical receiver 30 that is operable to detect the magnitude of the reflected energy 28 and to generate corresponding signals therefrom. The signals may then be communicated to an external recording or viewing device (not shown) to permit defects or discontinuities existing in the assembly 12 to be directly viewed.
The operation of the apparatus 10 of FIG. 1 will now be described further with reference to FIG. 2, which shows a graphical view of the amplitude and time domain behavior (or time-series) of the assembly 12 in response to a single pulse of the optical energy 24 applied at a time at an initial time t0. Reflected energy 28 stemming from the single pulse of optical energy 24 is generated at the connectors 18 and the defect 20 and propagates backwardly towards the terminal end 14, and is detected at respective times t1, t2 and t3 by the optical receiver 30. Once the detection times have been determined, the known velocity of propagation for the fiber optic segments 16 may be employed to determine the distances d1, d2 and d3 that correspond to the position of the connectors 18 and the defect 20.
Although desirable results have been achieved using the prior art apparatus, there is room for improvement. For example, the foregoing apparatus 10 may be suitable for a single, or widely-spaced optical pulses, it is less suited for built-in applications that are generally necessary for in-service, or built-in test equipment (BITE) applications in aircraft and the like. What is needed therefore is an apparatus and method for optically monitoring the condition of a fiber-optic assembly that are may be integrated into existing hardware.